1. Field of the Invention
The present invention relates to a method of forming contact holes when fabricating a semiconductor device.
2. Description of the Prior Art
A photolithographic process uses a KrF excimer laser that emits a KrF laser beam of 248 nm in wavelength as a light source for exposure, and a resist film of a resist which is relatively highly transmissive to a KrF laser beam in forming a pattern consisting of lines of a thickness in the range of 0.3 .mu.m to 0.4 .mu.m. Therefore, the dimensions of a pattern formed on the resist film are caused to deviate by the local variation of the thickness of the resist film due to steps formed in the base layer underlying the resist film and by a standing-wave effect resulting from the interaction of the transmissivity of the resist film and the reflectance of the base layer. Such deviation is more significant than that in g-line exposure or i-line exposure. To solve such a problem, a multilayer resist film is used or the resist film is formed over an antireflection film formed over the base layer. The photolithographic process uses a positive resist film highly absorptive to the exposure light to suppress the standing-wav e effect.
However, the use of a multilayer resist film or an antireflection film increases the number of steps of the photolithographic process. When the photolithographic process employs a positive resist film which is highly absorptive to the exposure light, pattern controllability is deteriorated greatly by the bulk effect.
For example referring to FIG. 1, if a positive resist film 11 which is highly absorptive to the exposure light is formed in a pattern having holes 12 and 13 on a stepped base layer 21, the holes 12 and 13 are tapered downward. Therefore, the respective diameters of the respective upper ends 14 and 15 of the holes 12 and 13 are greater than the respective diameters of the respective bottom ends 16 and 17 of the same holes. Accordingly, when the hole 12 is formed in a portion of the resist film 11 having a relatively small thickness and the hole 13 is formed in a portion of the resist film 11 having a relatively large thickness, the diameter of the bottom end 16 of the hole 12 is greater than that of the bottom end 17 of the hole 13. Thus, the diameter of the bottom end of a hole formed in the resist film is dependent on the thickness of the portion of the resist film in which the hole is formed. Hence it is difficult to adjust the sizes of holes formed in portions of the resist film formed over a stepped base layer and differing from each other in thickness.
If contact holes 23 and 24 are formed in the base layer 21 by using the resist film 11 having the holes 12 and 13 of such tapered shapes, the contact hole 23 corresponding to the hole 12, and the contact hole 24 corresponding to the hole 13 differ greatly from each other in diameter.